The invention relates to a drum brake device according to the preamble to claim 1, which is particularly provided as a wheel brake for a motor vehicle.
Drum brakes are known in and of themselves. The drum brakes of the known type have two brake shoes, which are disposed inside a brake drum. The brake shoes are pivotably supported at one end and at the other end, can be pressed outward against the brake drum, i.e. from the inside. The pressure of the braced brake shoes against the brake drum produces a braking moment on the brake drum.
One brake shoe in the known drum brakes is leading, i.e. a friction force between the brake drum and the brake shoe augments the contact force of the brake shoe against the brake drum. Another known design is the so-called duplex drum brake in which each of the two brake shoes respectively leads in one rotation direction of the brake drum. The action of the leading brake shoe, which augments the contact force of the brake shoe against the brake drum when the known drum brake is actuated, produces a boosting of the brake force as defined by an increase in a braking moment exerted on the brake drum when there is a given contact force of the brake shoe against the brake drum. Thus a powerful braking moment can be produced with a small contact force of the brake shoe against the brake drum. The boosting of the braking force, however, is limited by the fact that an excessive boosting causes a xe2x80x9cwedgingxe2x80x9d of the leading brake shoe in the brake drum, i.e. the leading brake shoe locks the brake drum due to the friction force exerted on the leading brake shoe by the rotating brake drum without the contact force of the brake shoe against the brake drum being increased. The boosting of the braking force must be chosen to be low enough that a locking of the brake drum due to the friction force it exerts on the leading brake shoe is reliably prevented in every operating state of the drum brake. This self-locking of the drum brake must be reliably prevented at every speed, every contact force of the brake shoe against the brake drum, when the drum brake is cold or hot, and in every wear state of a frictional brake lining of the brake shoes and brake drum.
Another disadvantage of the known drum brakes is that frictional brake linings of new brake shoes have to be adapted to the brake drum through actuation of the drum brake until they rest against the brake drum uniformly over their entire length in the circumference direction (so-called xe2x80x9cbraking inxe2x80x9d). If the brake drum expands due to heating, the frictional brake linings no longer rest against the brake drum uniformly over their entire length in the circumference direction, but rather the contact force increases in a limited circumference region of the brake shoe. In this circumference region, the frictional brake lining can overheat and xe2x80x9cvitrifyxe2x80x9d. Then the brake shoe has to be replaced.
The drum brake device according to the invention, with the features of claim 1, has a brake belt instead of brake shoes. The brake belt can flex in the radial direction of the brake drum and is disposed inside the brake drum in the vicinity of its inner circumference. In order to actuate the drum brake device according to the invention, a thrust is exerted on one end of the brake belt, approximately in the circumference direction, and as a result, this end of the brake belt is pressed against the brake drum from the inside. The thrust exerted on the end of the brake belt can also be exerted in an obliquely outward direction, i.e. with a component directed radially outward. During the actuation of the drum brake device, another end of the brake belt is braced in the circumference direction. The thrust exerted on the one end of the brake belt in the circumference direction extends over the entire length of the brake belt so that the brake belt is pressed against the brake drum from the inside over its entire length and thus exerts a braking moment on the brake drum.
It is sufficient if the brake belt extends over a part of the circumference of the drum brake device. Preferably, the brake belt extends over almost the entire circumference of the drum brake device, leaving a distance between the two ends of the brake belt that is small in comparison to the circumference of the brake drum.
The drum brake device according to the invention has the advantage that its brake belt, due to its flexibility in the radial direction of the brake drum, rests uniformly against the inside of the brake drum uniformly over its entire circumferential length, i.e. with a uniform contact force, which is directed radially outward. A localized overloading of the brake belt is prevented. Independent of a wear state of the brake belt and the brake drum, the brake belt always rests uniformly over its entire circumference against the brake drum. Even with a heating and expansion of the brake drum, the brake belt rests uniformly and therefore optimally against the brake drum over its entire length. A localized overheating due to a locally increased contact force of the brake belt against the brake drum is prevented.
When the drum brake device according to the invention is actuated, a friction force exerted by the rotating brake drum on the brake belt produces a force in the circumference direction on the brake belt, which augments the thrust exerted on the one end of the brake belt in order to actuate the drum brake device. The friction force exerted on the brake belt by the brake drum during actuation of the drum brake device increases a braking moment of the drum brake device. The drum brake device according to the invention has a high, controllable boosting of the brake force; a powerful braking moment can be produced with a small thrust (actuation force) on the one end of the brake belt. The term xe2x80x9ccontrollablexe2x80x9d is understood to mean that a locking of the brake drum, which is due the boosting of the brake force without an increase in the thrust exerted on the end of the brake belt, is prevented.
Advantageous embodiments and modifications of the invention disclosed in claim 1 are the subject of the dependent claims.
The brake belt of the drum brake device according to the invention can have a brake belt that is continuous in the circumference direction and flexible in the radial direction. The brake belt can, for example, be elastic or supple in the radial direction. In the circumference direction, the brake belt is preferably compressionally rigid so that it does not shorten or only shortens slightly when the drum brake device is actuated. The brake belt can also be embodied by affixing a frictional brake lining, which is continuous or discrete in the circumference direction, to a flexible support belt. In one embodiment of the invention, the brake belt is composed of links that are connected to each other in an articulating fashion (claim 2).